1. Field of the Invention
This invention relates to a process for the recovery of organic acids and other useful products such as nitric acid and catalysts contained in aqueous nitric acid solutions resulting from the oxidation of cyclohexanol and/or cyclohexanone to produce adipic acid.
2. Description of the Prior Art
The art is well aware of processes for producing useful acids, such as adipic acid, by oxidizing naphthenes, cycloaliphatic ketones or cycloaliphatic alcohols with nitric acid in the presence of metal compound oxidation catalysts. Such processes in general involve heating specific materials such as cyclohexane, cyclohexanol and/or cyclohexanone in nitric acid at about 40.degree. to 140.degree. C., generally utilizing nitric acid of about 20 to 90 percent strength, to produce a resulting oxidation mixture comprising the adipic acid together with small amounts of other dicarboxylic acids in admixture with the unused nitric acid and catalyst components. Generally, the art subsequently recovers a substantial amount of the desirable product, adipic acid, by cooling the solution and filtering off the crystallized adipic acid. Such processes for producing the adipic acid are known, for example, in the prior art from U.S. Pat. Nos. 2,791,566; 2,840,607; 3,338,959; 2,971,010; 2,439,513; and 2,557,282.
Adipic acid, of course, is an important intermediate for the production of nylon by subsequent copolymerization with hexamethylenediamine to produce a polyamide capable of being spun into a fiber having a number of desirable characteristics.
In the processes known to the art, however, very little attention has been paid to recovery of the valuable materials which can be further utilized by recycling the same to the nitric acid oxidation process such as unused nitric acid and catalytic components. Moreover, the other organic acids contained therein have usually been treated as waste materials.
In the process for nitric acid oxidation of cyclohexanol and/or cyclohexanone, significant amounts of succinic acid and glutaric acid are formed as byproducts in admixture with the adipic acid. Various well known schemes of crystallization, concentration and further crystallization have been used heretofore in industry to attempt separation thereof and usually a substantial amount of the adipic acid can be removed by these procedures. Ultimately, however, a mother liquor is obtained from these operations which contains succinic acid, glutaric acid, and a small amount of adipic acid in such proportions that further concentration and crystallization steps will yield only mixtures of these dibasic acids. In addition, as a result of the removal of the adipic acid by crystallization and removal of the water and nitric acid by volatilization, the concentration of the metal catalyst in this final mother liquor becomes relatively high. Hence, loss of these materials, particularly the catalytic components, provides a distinct economic disadvantage as the relatively high proportion of valuable and reusable products is lost.
It is clear, therefore, that a distinct need remains in the art for processes by which this nitric acid oxidation mother liquor may be processed so as to recover the valuable components contained therein in an economic and efficient manner.